A guess, the primary is 240V. 240-5% is 228V, just about right for 230V mains. 240-10% is 216V, close enough to 220V. Some units have a close specification on input voltage, for example RF power amplifiers need you to set the taps exactly right or the filaments on the tubes may fail.
Back in the old days Continental Europe ran on 220V and the UK ran on 240V (+/- the fiddle factor in each case). When the EU got involved they arranged for everyone to meet in the middle and now we all use a nominal 230V.